You may want to add some extra weight to the armboard. I have the rega armboard on my PT export and it has quite a large dense lead weighting block incorporated into the board. You should then try adjusting the subchasis springs until you get the desired height. One drawback may well be that the distance between arm and center spindle may be incorrect but you may be able to get a shop to sort as best as poss. Making a new one may be hard.